Pipettes are familiar tools in virtually all biological and chemical laboratories for the measurement of precise volumes of liquids.
The interior of a pipette must be clean and dry in order to preserve the accuracy of the volume and the content of the sample being measured. In commercial settings, the washing and drying of pipettes for reuse is generally considered too costly due to the labor and time involved. Therefore, disposable pipettes have come into popularity. Currently, such pipettes are assembled from two or three distinct parts. The principal part is a tube which is typically formed of a thermoplastic material through an extrusion process. The extruded tubing is cut into preselected lengths. Attached to a tube is a tapered tip. The tip is typically an injection molded part and is attached to the tube by means of a sonic weld or solvent seal. An optional mouthpiece may also be attached at the end of the tube opposite the tapered tip. Finally, the assembled pipette is printed with calibration marks.
Pipettes assembled according to this procedure suffer from several disadvantages. For example, the extrusion process often leaves the principal tubing with a cloudy appearance which makes reading of the calibration marks difficult. The assembly of the tip and optional mouthpiece to the tubing constitutes a secondary operation which is costly and time consuming. Further, sonic welds and solvent seals are known to fail in actual use due to a faulty weld or incomplete seal. There are often inaccuracies in measurement due to irregularities inside the tube at the junction of the tip and the tube. In addition, leakage may occur, causing inaccuracies in the volume transmitted. Finally, the tip may even separate from the tube while in use. Such a failure can ruin a test sample, especially in biological applications where samples are limited. In addition, failures of this type can cause a hazard to laboratory personnel where the liquid in the pipette consists of caustic reagents or virulent agents such as AIDS containing samples. Inaccuracies of volume can also result because the extrusion process does not yield a tube having a consistent inner diameter.